High-carbon chromium-nickel heat-resistant stainless steels are excellent in mechanical properties and oxidation resistance at high temperature over 800.degree. C and are widely used in high-temperature chemical engineering equipments. However, such steels generally have only limited workability, and therefore the tubular products of such steels are generally manufactured by centrifugal casting methods. The tubular products manufactured by the centrifugal casting methods have an unsound inner surface including casting defects, and the dimensional accuracy is low. The size of the products is also restricted.
The tubular or plate products of such steels may be manufactured in other ways. For instance, they can be manufactured by the usual hot working or hot- and cold-working processes such as extrusion and rolling from blooms made by the usual blooming processes. The products made by such processes are not as restricted in the shape and size as those made by the centrifugal casting methods. The impermissible drawback of the products made by such processes is that the high temperature creep rupture strength is markedly decreased as compared with the centrifugally cast products.
The reason is that, the carbide phase within the products manufactured by the usual hot working or hot- and cold-working exists as coarse M.sub.23 C.sub.6 -type carbide particles before use and the carbide becomes coarser during service at temperature over 800.degree. C, resulting in considerably low level of creep rupture strength at these temperatures.
A number of attempts have been made to improve the creep rupture strength of the products of high-carbon chromium-nickel stainless steels manufactured by usual hot- and cold-working. However, it has been well known that the creep rupture strength can be improved only to a small extent by a heat treatment process in which the said products are subjected to solution treatment and subsequent aging.
There is an alternative heat treating process to improve the creep rupture strength of hot- and cold-worked products of high-carbon chromium-nickel stainless steels by giving a microstructure where a continuous carbide precipitation is present along the grain boundaries by applying the said heat treating process, in which the said products are heated to a temperature range between 1150.degree. C to the solidus line with subsequent slow cooling to 950.degree. C or higher over a period of from 5 sec. to 1 hour and thereafter rapid cooling. By applying such heat treatment the creep rupture strength of wrought high-carbon chromium-nickel stainless steels are said to be increased at elevated temperatures where grain boundary sliding is a dominant factor controlling the creep rupture strength of the said steels, because grain boundaries are strengthened by continuous precipitation of M.sub.23 C.sub.6 -type carbide. However, this heat treating process has some difficulties from the practical standpoint of view. For example, it will take a rather long period probably not less than 60 min., because it is well known that rows of coarse particles of M.sub.23 C.sub.6 -type carbide can not readily be redissolved in a short time at a temperature below about 1300.degree. C.